Automate merging of statements

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Current way of merging of duplicated statements involves some manual work. In many cases it could be automated. Currently the process of merging is as follows:

1. Select the statement this error message "This statement is the same as the previous one - 'label'" appeared for by clicking the checkbox left to the statement. This must be the statement right above the error message. And make sure this is the only statement selected.
2. Click "Merge two similar statements" button. This button looks like two lines merging into a single line (like Y).
3. A dialog should appear where you will have possibility to select what statement you want to continue using. (you selected one statement, metamath-lamp finds another duplicated statement automatically)
   This way one of the duplicated statements will be removed automatically and metamath-lamp will update all the justifications which used the removed statement so they will use the statement you chose to continue using.

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The "auto merge" feature should not remove the G step.

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Handle this case too:

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This is implemented on dev.

Automatic merging of similar steps is optional. There is a global setting "Merge similar steps automatically". It is Off by default. It can merge only in simple cases: 1) both similar steps must be P or G (not H); 2) one of them must have a non-empty justification and another must have an empty justification. Only in that case the one with an empty justification will be removed and all references to it will be replaced with the label of the remaining step. If automatic merging is not possible then the manual merging is still available and works as before (the Merge icon).

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One more case is added which the automatic merging can handle - if both statements have same justification. In that case the upper one will remain and the lower one will be removed.

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This is available in version 16